1. Field of the Invention
The present invention relates to a position measuring device for determining the position of two elements, which are movable with respect to each other, having a scanning unit, which transmits data to an evaluation unit via an interface, and having a voltage source for the scanning unit. The present invention also relates to a method for the start-up of a position measuring device.
2. Discussion of Related Art
Such position measuring devices are for example used in connection with machine tools in order to determine the absolute position of a workpiece, or its position with respect to a tool.
A position measuring device of the type mentioned has been described in DE 43 42 377 A1 of Applicant, which corresponds to U.S. Pat. No. 5,687,103 the entire contents of which are incorporated herein by reference. A position measuring device is completely integrated into a scanning unit, and the signals of a detector scanning a graduation are digitized and position data are calculated from this. The data, which contain for one the position data from the actual position measurement, and also further information regarding the position measuring device itself, by which a processing unit can be adapted to the position measuring device, are transmitted from the position measuring device to the processing unit via an interface.
One problem regarding this and similar position measuring devices is the provision of an assurance of a supply voltage required for the dependable operation of the scanning unit of the measuring device. If this voltage is not high enough, for example because the user has employed an electrical cable of too narrow a cross-section and therefore too high a cable resistance, malfunctions of the scanning unit can occur, which result in a wrong position determination. Control commands issued on the basis of such erroneous position determinations can lead to damage of the tool, or the workpiece in machine tools, for example. But the supply voltage can also be so low that it is not possible to initiate the position measuring device at all.
A method for the transmission of multiple serial signals, which are employed for a position control and regulation of a motor, is described in DE 44 22 056 A1, which corresponds to U.S. Pat. No. 5,760,707 the entire contents of which are incorporated herein by reference. The transmission of a control bit for the supply voltage, which is supplied by a voltage drop detector, is disclosed there. However, it is disadvantageous that in case of too low a supply voltage after switch-on, the interface itself, which is intended for transmitting this control bit, does not operate. Therefore, an evaluation unit, which receives the control bit, is not even informed of the status xe2x80x9csupply voltage below normal.xe2x80x9d The user is not provided any information for the search for the fault.
For monitoring the supply voltage when switching on any arbitrary electronic component and to prevent an insufficient supply to them, it is known from JP 9 091045 A2 to determine the internal resistance of the voltage source from the voltage drop when an electronic component, whose current consumption is known, is switched on and, knowing the current consumption of further electrical components, to activate them only if the supply voltage would not fall below a threshold value.
It is an object of the present invention to disclose a position measuring device with a scanning unit, wherein a check of the voltage supply required for the dependable operation of the scanning unit already takes place when it is switched on.
This object is attained by a position measuring device for determining the position of two elements, which are movable with respect to each other, the device including a scanning unit that includes one or more individual electrical components, a voltage source and a voltage monitoring unit that selectively supplies the one or more individual electrical components with a voltage.
It is a further object of the present invention to disclose a method for the start-up of the scanning unit of a position measuring device, which permits the check of the supply voltage required for the dependable operation of the scanning unit already when it is switched on.
This object is attained by a method for the start-up of a position measuring device including a scanning unit, the method includes switching on a position measuring device that includes a scanning unit, subsequent to the switching on, performing a check of a supply voltage of the scanning unit supplied by a voltage source and activating one or more electrical components in the scanning unit, provided a sufficient supply voltage for the activating has been determined during the performing of the check.
Following the switch-on of the position measuring device, initially only the interface between the scanning unit and the evaluation unit, as well as a voltage monitoring unit, are provided with a voltage. The voltage monitoring unit then determines the size of the internal resistance of the voltage supply of the scanning unit and calculates that supply voltage from this, which should be available in the scanning unit after all electrical components have been switched on. Only if this value is sufficient for a dependable operation of the scanning unit, is the latter completely activated, otherwise an error message is issued via the interface. The user of such a system has the great advantage that, when the scanning unit is insufficiently supplied, he is not confronted with erroneous measurements, which are hard to detect, and he saves time when searching for the errors in case only a wrong cable has been used for the voltage supply, or if bad contact resistances occur at the cable connections.
Further advantages, as well as details of the present invention, ensue from the following description of a preferred embodiment represented in the drawings.